When a screen is illuminated by coherent light such as laser, due to coherence of the laser and optical roughness of the screen surface, speckle noise pattern is observed by observer's eyes, which seriously affects the display quality of the image and covers useful information. Therefore, how to reduce the speckles has been a research hotspot in the fields of optical instruments and display technologies. In view of the current research, many methods used to reduce speckle have been developed, which roughly can be classified as the following: i) Controlling the coherence time of the laser source to reduce speckle, wherein the principle is to generate boiling speckle by adjusting the laser wavelength (or frequency) or by using multiple independent laser source, most solutions which succeed to reduce speckles to meet practical requirements by controlling the laser coherence time are basically based on multiple light sources superimposed; and ii) Controlling the spatial coherence of the laser beams to reduce speckles, wherein the basic principle is to adjust the phase distribution of the primitive lightwave in the laser beam, thereby changing the spatial distribution of the speckle pattern, a plurality of speckle images are superimposed during the integration time of the human eye to obtain an image with uniform light distribution, thus achieving the purpose of reducing speckles. There are many methods to change the spatial distribution of laser beam, such as rotating a diffuser, vibrating a screen or a binary phase diffuser with Hadamard matrix configures, high frequently vibrating an optical fiber and so on. All the above methods require mechanical vibration, even the high-frequency or large amplitude vibration, or require integration of multiple light sources, with the disadvantages of complex structure, easy to damage, high cost not only, but also speckle reduction being ineffective.
There are technical solutions with no mechanical vibration. For example, China Patent No. 200820122639.7 discloses a scattering-based decoherence shimming device, which requires the use of scattering media containing particles having a diameter less than one-tenth of the wavelength of the incident light, in order to achieve Rayleigh scattering of the incident laser. In this patent, the aqueous solution of an inorganic salt or an organic alcohol (such as NaCl, KCl, KNO3 or ZnSO4) is used as the scattering medium. As the aqueous solution of the inorganic salt or the organic alcohol is present in the form of hydrated ions or macromolecules, the dimensions of which are much smaller with respect to the laser wavelength, Rayleigh scattering occurs for the incident laser, which results in incident laser splitting and conducting within the light guide, in order to reduce the coherence of the incident laser to reduce speckles, while taking advantage of light mixing effect of the light guide, performing the uniformity of the split light to shim decoherence. However, in the experiment according to the teachings of the above technical means, at room temperature, 50 mm in length of a light guide filled with a saturated NaCl aqueous solution is used to reduce speckles, and the result is shown in FIG. 1, which illustrates the speckle contrast of 70%, and almost no effect of speckle reduction.